Motor armature current limit system

ABSTRACT

The system allows maximum acceleration and deceleration of a constant field motor and prevents commutator flashing during forward and reverse motoring and plugging operations. Positive and negative standstill current limits and acceleration and deceleration current limits are independently adjustable for optimum motor operation.

I United States Patent [151 3 649 897 9 Messick Mar. 14, 1 972 [541MOTOR ARMATURE CURRENT LIMIT References Cited SYSTEM UNITED STATESPATENTS [72] Inventor: Mikel Jay Messick, Chagrin Falls, OhlO 3,508,1324/1970 Peterson ..3l8/308 X [73] Assignee: Square D Company, Park Ridge,Ill.

Primary Examiner-J. D. Miller [22] Flled' 1970 Assistant Examiner-RobertJ. Hickey [2]] Appl. No.: 83,073 Attorney-Harold J. Rathbun and RichardT. Guttman [57] ABSTRACT 318/4335; The System allows maximumacceleration and deceleration of 58] i 269 a constant field motor andprevents commutator flashing during forward and reverse motoring andplugging operations. Positive and negative standstill current limits andacceleration and deceleration current limits are independentlyadjustable for optimum motor operation 7 Claims, 6 Drawing FiguresCONTROL REE E SUMMING gg f OPERATING GENERATOR AMPL'F'ER AMPLIFIER N9 l6CURRENT LIMIT MOTOR ARMATURE CURRENT LIMIT SYSTEM This invention relatesto motor armature current limit circuits for direct current motors, andmore particularly to such a circuit which will permit maximumrates ofacceleration and deceleration while protecting the motor againstcommutation difficulties.

Many industrial operations require the use of high speed motors whichcan accelerate and decelerate at a rapid rate so that no time is wastedbetween operations of the associated machinery. If an automatic motorcontrol system is used, it will call for the application of a highcurrent to effect the change in speed. Such operation may bring themotor beyond its commutation limit and produce arcs which pit and wearthe motor brushes and commutator bars. Operation beyond the commutationlimit can cause heavy arcing between the brushes and across thecommutator surfaces severely damaging or destroying the motor.Therefore, armature current must be limited during all phases of motoroperation.

Safe motor operation within the armature current commutation range isgenerally achieved by circuitry which fixes the maximum value ofstandstill armature current. Although the safe commutating currentdiminishes as armature speed increases, motor counter electromotiveforce (emf) causes the current to decrease at a greater rate so that itstays well within safe limits. However, the motor consequently operatesat less than its maximum acceleration rate and is unable to attain itsmaximum speed. During motor plugging, i.e., slowing of the motor by theapplication of reverse torque, the counter emf enhances the armaturecurrent so that the established standstill current limit is insufficientto prevent commutation breakdown. Bidirectional motor operation byarmature control requires current limit circuitry operative with currentpassing through the armature in either direction.

It is an object of the present invention to provide a circuit forlimiting the armature current of a variable speed direct current motor,the circuit being responsive to a control signal input and providingoptimum speed and torque characteristics through forward and reversemotoring and plugging modes of motor operation.

This and other objects of the present invention can be best describedwith reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a motor control system incorporating thecurrent limit circuit of the present invention;

FIG. 2 is a schematic wiring diagram of the current limit circuit of thepresent invention;

FIG. 3 is a graph of armature current vs. motor speed showing a motorcurrent limit envelope;

FIG. 4 is a series of graphs labeled A, B, and C showing voltagerelations involved in the operation of the current limit circuit of thepresent invention;

FIG. 5 is a graph showing armature current and motor speed relationsinvolved in the operation of the current limit circuit of the presentinvention; and

FIG. 6 is a graph showing voltage and motor speed relations involved inthe operation of the current limit circuit of the present invention.

Referring to FIG. 1, a current limit circuit 11 in accordance with thepresent invention is shown in connection with an automatic controlsystem for a DC motor having a fixed field. The control system suppliesa control signal to an operating circuit 12 which responds to provide anoperating voltage to an armature 14 of the motor, the magnitude andpolarity of the applied operating voltage determining the magnitude anddirection of current through the armature 14. A preferred operatingcircuit is disclosed in my application Ser. No. 83,074 filedconcurrently herewith and assigned to the assignee of the presentinvention.

The armature 14 is mechanically coupled to a tachometergenerator 15which generates a DC voltage having a magnitude directly proportional tothe motor shaft speed and a polarity depending on the direction of motorrotation. A control reference signal generator 16 produces a DC voltagehaving a magnitude indicative of the desired motor speed and having apolarity indicative of the desired direction of motor torque. Forexample, a reference signal of positive voltage could call for a forwardmotor operation and a reference signal of negative voltage could callfor reverse motor operation. The polarity of the voltage signal producedby the tachometer-generator 15 would then be negative during forwardoperation of the motor and positive during reverse operation of themotor.

The output voltages of the control reference signal generator l6 and thetachometer-generator 15 are transmitted to a summing amplifier 17 whichcombines the signals to produce an error signal which is in turntransmitted to a control signal amplifier 19. The error signal isamplified to provide a control signal from the amplifier 19 whichdetermines the operation of the operating circuit 12 as described in theaforementioned patent application. The magnitude of this control signalcon trols the voltage applied to, and resulting current through, thearmature 14 and the polarity of the control signal determines whether aforward torque (positive signal) or a reverse torque (negative signal)should be applied.

The current limit circuit 11 illustrative of this invention will now bedescribed with reference to FIGS. 1 and 2. The error signal output ofthe summing amplifier 17 is transmitted to the current limit circuit 11at a signal input terminal 20. The voltage signal generated by thetachometer-generator 15 enters the current limit circuit 11 at thetachometer input terminal 21. The current limiting signals produced bythe current limit circuit 11 are transmitted to the control signalamplifier 19 via signal output terminals 22 and 24. A common terminal 25connects common conductors 26 and 27 to the control system common.

A conductor 29 connects the signal input terminal 20 through an inputresistor 30 to the input of an operational amplifier 31. An inputresistor 32 connects the conductor 29 to the input of an operationalamplifier 34. The amplifiers 31 and 34 are connected to serve as signalinverting amplifiers; this means that a positive input voltage producesa negative output voltage and that a negative input voltage produces apositive output voltage.

An adjustable resistance means such as a potentiometer 35 having amovable contact 35c is connected between a negative voltage source 36and the common conductor 26. The movable contact 350 is connectedthrough an input resistor 37 to the input of the amplifier 34. Apotentiometer 39 having a movable contact 390 is connected between apositive voltage source 40 and the common conductor 27. The movablecontact 39c is connected through an input resistor 41 to the input ofthe amplifier 31.

The tachometer input terminal 21 is connected to one end of a pluggingpotentiometer 42, having a movable contact 42c, and to one end of amotoring potentiometer 44, having a movable contact 440. The other endsof the potentiometers 42 and 44 are connected to the common conductor27. A conductor 45 connects the movable contact 42c through aunidirectional current conduction means, such as a diode 46, and aninput resistor 47 to the input of the amplifier 34 and the conductor 45is in turn connected by a conductor 49 through a diode 50 and an inputresistor 51 to the input of the amplifier 31. The movable contact 440 isconnected by a conductor 52 through a diode 54 and an input resistor 55to the input of the amplifier 34 and the conductor 52 is connected by aconductor 56 through a diode 57 and an input resistor 59 to the input ofthe amplifier 31.

A pair of conductors 62 and 64 connect the amplifiers 34 and 31 to theoutput terminals 22 and 24, respectively. A feedback resistor 65 and aclamping diode 66 are connected in parallel across the amplifier 34 anda feedback resistor 67 and a clamping diode 69 are connected in parallelacross the amplifier 31.

Operation of the current limit circuit 11 will now be described. Thearmature 14 is to carry currents for operation in forward and reversemotoring and plugging modes and must be protected from commutationfailure in each of the four modes. A desirable motor current limitenvelope such as shown in FIG. 3 provides maximum acceleration anddeceleration with freedom from commutation failure and has a forwardmotoring quadrant 70, a plugging from forward quadrant 71, a reversemotoring quadrant 72 and a plugging from reverse quadrant 73. Thecontour of the envelope is made to approximate maximum armature currentpermissible during each mode of motor operation. The maximum permissiblearmature current, as determined by the commutating ability of the motor,decreases as motor speed increases so that the current limit circuitmust provide means to keep the armature current just within theperimeter of the envelope under all conditions for optimum motoroperation.

When the reference signal to the signal input terminal and thetachometer signal to the tachometer input terminal 21 (FIG. 2) are bothzero, the only input to the amplifier 34 is a negative bias voltage fromthe negative voltage source 36 supplied through the potentiometer 35.Since the amplifier 34 is connected to serve as a signal inverter, thenegative input tends to drive the output of the amplifier 34 positivewith a signal gain equal to the ratio of the resistance of the feedbackresistor 65 to that of the input resistor 37. However, with a positiveamplifier output, the clamping diode 66 shorts out the feedback resistor65 thereby clamping the output of the amplifier 34 at a near-zerovoltage of a magnitude determined by the forward drop of the diode 66.

Similarly, the only input to the amplifier 31 is a positive bias voltagefrom the positive voltage source 40 supplied through the potentiometer39. Since the amplifier 31 is connected to serve as a signal inverter,the positive input tends to drive the output of the amplifier negativewith a signal gain equal to the ratio of the resistance of the feedbackresistor 67 to that of the input resistor 41. However, with a negativeamplifier output, the clamping diode 69 shorts out the feedback resistor67 thereby clamping the output of the amplifier 31 at a near-zerovoltage of a magnitude determined by the forward drop of the diode 69.The negative output of the amplifier 31 is preferably equal in magnitudeto the positive output of the amplifier 34 under zero input conditions.

When the armature 14 is not rotating, so that the tachometer inputvoltage is zero, and a positive reference signal is applied to thesumming amplifier 17 (FIG. 1), this signal is applied to the signalinput terminal 20 of the current limit circuit 11. The signal is summedwith the bias voltages at the amplifiers 34 and 31 and subtracts fromthe bias voltage applied to the amplifier 34 and adds to the biasvoltage applied to the amplifier 31. The output of the amplifier 31remains clamped, but when the magnitude of the positive signal inputvoltage exceeds the bias voltage from the source 36, the summed input tothe amplifier 34 becomes positive and the output becomes correspondinglynegative. The diode 66 no longer clamps the output of the amplifier 34and its output increases negatively at the same rate at which the signalinput voltage increases positively so that the sum of the input voltagesto the control signal amplifier 19 becomes constant. Similarly, anegative reference signal clamps the output of the amplifier 31 andpermits an increasing negative voltage from the amplifier 34. Thus, apositive and a negative standstill current limit, shown at 75 and 76,respectively, in FIG. 3, are defined for the armature 14.

A voltage signal output from the summing amplifier 17 (FIG. 1), such asthat shown in FIG. 4A, would thus cause the current limit circuit 11 toproduce a voltage output such as I that shown in FIG. 4B andhereinbefore explained. These voltages signals would be summed by thecontrol signal amplifier 19 to produce the clamped signal shown in FIG.4C. It can readily be seen from the foregoing explanation thatadjustment of the potentiometer 35 determines the positive standstillcurrent limit while adjustment of the potentiometer 39 determines thenegative standstill current limit.

When the motor is operating in the forward direction and the output ofthe control reference signal generator 16 is positive and greater inmagnitude than the negative output of the tachometer generator 15, themotor operates in its forward motoring mode (quadrant 70 in FIG. 5). Asthe motor speed increases, the counter emf of the motor increases sothat, even if the applied terminal voltage remains constant, thearmature current decreases. The drop in armature current with increasingspeed is shown by a curve 77 in FIG. 5. It can be seen that the naturaldecline in annature current from the standstill current limit is sogreat that the motor would not even approach its maximum speed. Sinceoptimum operation of the motor requires that the armature current bemaintained along the current limit curve 79, the applied terminalvoltage must be increased to partially counteract the counter emf of themotor. The counter emf, shown by a curve 82 in FIG. 6, increases withthe motor speed so that an applied terminal voltage having a constantmagnitude would produce the armature current-speed curve 77 in FIG. 5.To maintain armature current along the perimeter 79 of the current limitenvelope when maximum current is called for, the terminal voltage mustbe increased in a manner such as shown in FIG. 6 by a curve 84. Themotor terminal voltage is increased at a rate less than the increase incounter emf, thereby producing a decrease in the net motor voltage whichcauses the annature current to decrease along the perimeter of thedesired envelope. It should be noted, however, that whenever the signalinput calls for a current less than the permissible limit, the currentlimit circuit will cause no changes in the control signal.

The signal input terminal 20 receives a positive voltage and thetachometer input terminal 21 receives a negative voltage during forwardmotoring operation. The positive signal input voltage clamps the outputof the amplifier 31 and tends to overcome the bias voltage applied tothe amplifier 34, as previously explained, causing application of acurrent limiting output. The negative tachometer input voltage isapplied through the potentiometer 42 to the diodes 46 and 50 and throughthe potentiometer 44 to the diodes 54 and 57. The diodes 46 and 57 areso poled that the voltage is blocked and the diodes 50 and 54 are sopoled that the voltage is transmitted to the amplifiers 31 and 34,respectively. The magnitude of the negative tachometer input voltage isinsufficient to unclamp the amplifier 31 and thus has no efiect on itsoutput. However, the negative signal does affect the output of theamplifier 34; it combines with the positive signal input voltage and thenegative bias voltage to the input to the amplifier 34.

The combination of the negative tachometer input signal with thenegative bias from the potentiometer 35 permits a larger positivevoltage to be presented at the signal input terminal 20 before a currentlimiting output is generated. This means that, for a constant positiveinput signal, the current limit signal at the output terminal 22decreases in magnitude as the tachometer input voltage increases.Therefore, as the motor counter emf increases as shown by the curve 82in FIG. 6, the applied terminal voltage increases as shown by the curve84. The rate of increase of applied terminal voltage is that required toconform the current taper to the current limit curve 79 in FIG. 5.Conformity of the motoring current curve to the desired current limitenvelope during forward motoring operation may be regulated byadjustment of the motoring potentiometer 44.

If, during forward operation of the motor, it is desired to reduce themotor speed or reverse the direction of annature rotation, the output ofthe control reference signal generator 16 becomes negative thus callingfor a reversal of the polarity of the applied terminal voltage forplugging the motor. Should the terminal voltage be reversed, the counteremf would have the same polarity as the terminal voltage and a largecurrent pulse would be sent through the motor, as shown by a curve 80 inFIG. 5. To keep the armature current along the curve 81 in FIG. 5, theapplied terminal voltage must be kept positive and its magnitudemaintained at a lesser value than that of the counter emf. As motorspeed decreases, terminal voltage is decreased, as shown by a curve 85in FIG. 6, remaining positive until a negative terminal voltage can beapplied without producing a current beyond the commutation limit.

During plugging from forward operation, the voltage at the signal inputterminal 20 is negative and the voltage at the tachometer input terminal21 is negative. The negative signal input voltage clamps the output ofthe amplifier 34 so that only the amplifier 31 can function. Thetachometer input is fed through the potentiometer 42 to the diodes 46and 50 and through the potentiometer 44 to the diodes 54 and 57. Thediodes 46 and 57 block the negative voltage and the input through thediode 54 to the amplifier 34 is insufficient to unclamp its output.Therefore, the negative tachometer voltage is summed through the inputresistor 51 with the negative input signal voltage and the positive biasvoltage for input to the amplifier 31. The added tachometer signalpermits a smaller negative voltage at the signal input terminal 20 togenerate a current limiting output. Thus, for a constant negative inputsignal, the current limit signal at the output terminal 24 is greatestdun'ng high-speed operation which produces a higher tachometer inputvoltage. During high-speed plugging, the positive output of theamplifier 31 and thus of the current limit circuit 11 is greater inmagnitude than the negative output of the summing amplifier 17 so thatthe output of the control signal amplifier 19 remains positive. As motorcounter emf decreases, the applied terminal voltage decreases finallybecoming negative, as shown by the curve 85 in FIG. 6, so that thearmature current increases with decreasing motor speed at a rate notgreater than that shown by the curve 81 in FIG. 5. Conformity of theplugging current curve to the current limit envelope during forwardoperation may be regulated by adjustment of the plugging potentiometer42.

Similar operation occurs during the reverse motoring and plugging fromreverse modes of motor operation. During reverse motoring, a negativevoltage is applied at the signal input terminal 20 and the tachometerinput voltage is positive. Therefore, the output of the amplifier 34 isclamped and the amplifier 31 produces a current limiting signal alteredto compensate for the effects of counter emf by the positive tachometersignal applied through the motoring potentiometer 44 and the diode 57.Again the potentiometer 44 provides current taper adjustment.

During plugging from reverse operation, a positive input signal voltageclamps the output of amplifier 31 and unclamps the output of amplifier34. The positive tachometer voltage applied through the pluggingpotentiometer 42 and diode 46 alters the current limiting signal tocompensate for the motor counter emf. The potentiometer 42 providesadjustment. The ohmic values of the input resistors 47, 55, 51 and 59may be adjusted or selected to create appropriate gain characteristicsto provide the versatility required of the current limit circuit 11 forproper control during all modes of motor operation.

lclaim:

1. A control system for a motor having an armature, said control systemcomprising means combining a control reference voltage and a speedreference voltage to provide a control voltage of selectable magnitudeand polarity, means responsive to the magnitude and polarity of anoperating voltage to transmit a corresponding current through thearmature, current limit means responsive to a predetemiined value ofsaid control voltage to produce a current limit voltage having apolarity opposite the polarity of said control voltage and a magnitudesubstantially equal to the amount by which the magnitude of said controlvoltage exceeds said predetermined value, and means to combine saidcontrol voltage and said current limit voltage to produce said operatingvoltage, whereby current limit is provided for the armature atstandstill and all speeds of the motor.

2. A control system as in claim 1 wherein said current limit meanscomprises first and second signal inverting amplifier means having inputand output terminals, negative biasing means for applying apredetermined negative biasing voltage to the input terminal of saidfirst amplifier means to produce a positive amplifier output, positivebiasing means for applying a predetermined positive biasing voltage tothe input terminal of said second amlplifie r means to produce anegative amplifier output, a first c ampmg means for limiting thepositive output of said first amplifier means, a second clamping meansfor limiting the negative output of said second amplifier means, and afirst input means for applying said control voltage to the inputterminals of said first and second amplifier means.

3. A control system as in claim 2 wherein each of said first and secondamplifier means has a feedback resistor and wherein said first clampingmeans comprises a unidirectional current conduction means connected inparallel with said first feedback resistor and said second clampingmeans comprises a unidirectional current conduction means connected inparallel with said second feedback resistor.

4. A control system as in claim 2 including tachometer means coupled tothe armature to produce said speed reference voltage, and wherein saidcurrent limit means includes a second input means to apply said speedreference voltage to the input terminals of said first and secondamplifier means.

5. A control system as in claim 4 wherein the system provides motoringand plugging modes for said motor, and wherein said current limit meansincludes speed reference adjustment means to permit independentadjustment for said speed reference voltage for each of said motoringand plugging modes.

6. A control system as in claim 5 wherein said speed referenceadjustment means comprises first and second adjustable resistance means,said first adjustable resistance means being connected to the inputterminal of said first amplifier means through a first unidirectionalcurrent conduction means and to the input tennina] of said secondamplifier means through a second unidirectional current conductionmeans, said second adjustable resistance means being connected to theinput terminal of said first amplifier means through a thirdunidirectional current conduction means and to the input terminal ofsaid second amplifier means through a fourth unidirectional currentconduction means, and wherein said first and fourth unidirectionalcurrent conduction means are poled to transmit one polarity of saidspeed reference voltage and said second and third unidirectional currentconduction means are poled to transmit the other polarity of said speedreference voltage.

7. A control system as in claim 2 wherein each of said negative andpositive biasing means includes adjustable resistance means.

1. A control system for a motor having an armature, said control system comprising means combining a control reference voltage and a speed reference voltage to provide a control voltage of selectable magnitude and polarity, means responsive to the magnitude and polarity of an operating voltage to transmit a corresponding current through the armature, current limit means responsive to a predetermined value of said control voltage to produce a current limit voltage having a polarity opposite the polarity of said control voltage and a magnitude substantially equal to the amount by which the magnitude of said control voltage exceeds said predetermined value, and means to combine said control voltage and said current limit voltage to produce said operating voltage, whereby current limit is provided for the armature at stand-still and all speeds of the motor.
 2. A control system as in claim 1 wherein said current limit means comprises first and second signal inverting amplifier means having input and output terminals, negative biasing means for applying a predetermined negative biasing voltage to the input terminal of said first amplifier means to produce a positive amplifier output, positive biasing means for applying a predetermined positive biasing voltage to the input terminal of said second amplifier means to produce a negative amplifier output, a first clamping means for limiting the positive output of said first amplifier means, a second clamping means for limiting the negative output of said second amplifier means, and a first input means for applying said control voltage to the input terminals of said first and second amplifier means.
 3. A control system as in claim 2 wherein each of said first and second amplifier means has a feedback resistor and wherein said first clamping means comprises a unidirectional current conduction means connected in parallel with said first feedback resistor and said second clamping means comprises a unidirectional current conduction means connected in parallel with said second feedback resistor.
 4. A control system as in claim 2 including tachometer means coupled to the armature to produce said speed reference voltage, and wherein said current limit means includes a second input means to apply said speed reference voltage to the input terminals of said first and second amplifier means.
 5. A control system as in claim 4 wherein the system provides motoring and plugging modes for said motor, and wherein said current limit means includes speed reference adjustment means to permit independent adjustment for said speed reference voltage for each of said motoring and plugging modes.
 6. A control system as in claim 5 wherein said speed reference adjustment means comprises first and second adjustable resistance means, said first adjustable resistance means being connected to the input terminal of said first amplifier means through a first unidirectional current conduction means and to the input terminal of said second amplifier means through a second unidirectional current conduction means, said second adjustable resistance means being connected to the input terminal of said first amplifier means through a third unidirectional current conduction means and to the input terminal of said second amplifier means througH a fourth unidirectional current conduction means, and wherein said first and fourth unidirectional current conduction means are poled to transmit one polarity of said speed reference voltage and said second and third unidirectional current conduction means are poled to transmit the other polarity of said speed reference voltage.
 7. A control system as in claim 2 wherein each of said negative and positive biasing means includes adjustable resistance means. 